1. Field of the Invention
The invention relates to a photosensitive compound and a photosensitive polymer derived from the compound. It also relates to a photoaligning agent including the polymer, a liquid crystal alignment film prepared from the photoaligning agent, and an optical film or a liquid crystal display device containing the liquid crystal alignment film.
2. Related Art
A liquid crystal display device has been used for liquid crystal display apparatuses from the beginnings of monitors of notebook personal computers and desktop personal computers to the end of viewfinders of video cameras, projection displays, television sets and so forth. It is also utilized for optoelectronic-related devices such as light print heads, light Fourier transform devices and light valves. Use of nematic liquid crystals in a display device is the mainstream of conventional liquid crystal display devices. The liquid crystal display devices having a TN (Twisted Nematic) mode, a STN (Super Twisted Nematic) mode and TFT (Thin-Film-Transistor) mode have been put to practical use; in the TN mode, the direction of orientation of liquid crystals located near to one substrate and the direction of orientation of liquid crystals located near to the other substrate are twisted at an angle of 90 degrees; in the STN mode, the directions of orientation is normally twisted at an angle of 180 degrees or more; and in the TFT, thin-film-transistors are used.
However, in these liquid crystal display devices, the viewing angle, in which images can be seen properly, is narrow, and thus there are cases where brightness or contrast decreases when viewed diagonally, and brightness inversion occurs in a halftone image. In recent years, the subject of the viewing angle has been improved by use of a TN mode in combination with an optical compensation film, a MVA (Multi-domain Vertical Alignment) mode (see Patent document No. 1) using homeotropic orientation together with technology of protuberance, IPS (In-Plane Switching) mode utilizing a lateral electric field type (see Patent document No. 2), and so forth.
The development of technology of the liquid crystal display devices is attained not only by an improvement of the driving mode or the device structure, but also by an improvement of members used for making the display device. Among members used for making the display device, in particular, a liquid crystal alignment film is one of important elements related to the display quality of the liquid crystal display device, and the role of the liquid crystal alignment film becomes more important year after year as the quality of the display device is increased.
In the liquid crystal alignment film, it is required that the molecular orientation of liquid crystals is controlled for the purpose of uniform display characteristics of the liquid crystal display device. That is to say, it is required that the liquid crystal molecules are uniformly arranged in one direction, and a pretilt angle, which is a definite slope from the surface of a substrate, is generated.
An optical compensation film or a phase retardation film is used to realize an improvement of contrast or an increase of the range of a viewing angle in an image display apparatus, and examples are a stretched film having optical anisotropy or a film prepared by alignment, and then by polymerization of a polymerizable liquid crystal compound. A liquid crystal alignment film is also used to align this polymerizable liquid crystal compound. The liquid crystal alignment film, by which the direction of liquid crystal molecules on a substrate is uniformly arranged, is utilized in a variety of steps in the production of a liquid crystal display device, and technique of the film becomes important and necessary.
The liquid crystal alignment film is prepared from a liquid crystal aligning agent. The liquid crystal aligning agent that is used mainly at present is a solution in which a polyamic acid or a soluble polyimide is dissolved in an organic solvent. After such a solution has been applied to the surface of a substrate, a polyimide-based liquid crystal alignment film is formed by means such as heating. Although a variety of liquid crystal aligning agents other than those using the polyamic acid have been studied, they are hardly put to practical use in view of heat resistance, chemical resistance (liquid crystal resistance), coating properties, ability to orient liquid crystals, electrical characteristics, optical characteristics, display characteristics and so forth.
For industrial use, a rubbing method that is conveniently and rapidly possible to treat a wide area has been widely used for alignment treatment. The rubbing method is a treatment in which the surface of a liquid crystal alignment film is rubbed in one direction by use of a cloth planted with a fiber such as nylon, rayon and polyester, and the method makes it possible to orient liquid crystal molecules uniformly. However, the rubbing method mostly generates dusts or static electricity, and thus concerns are alignment defects or an effect to liquid crystal device.
In recent years, another method for controlling orientation of liquid crystals, by which the rubbing method is replaced, has been developed. In a photoalignment method where alignment treatment is carried out by irradiation with light, a variety of alignment by means such as photodecomposition, photoisomerization, photodimerization and phtobridging have been proposed (see, Non-patent document No. 1, Patent document No. 3 and Patent document No. 4). The photoalignment method is non-contact, which is different from the rubbing method, and the generation of dusts or static electricity is theoretically less than that in the rubbing treatment.
The molecular alignment of a monomolecular layer in liquid crystals that is contacted to the liquid crystal alignment film can be controlled by using a liquid crystal alignment film having an excellent orientation ability in which the alignment is treated by the photoalignment method. Accordingly, an improvement of the film for a liquid crystal display device can be expected.